An ultrasonic diagnostic apparatus is a type of imaging apparatus generally employed in a medical field, together with an MRI system and a CT scanner, and characterized by compact in size, a high spatial and time resolution, and the like. In recent years, along with the widespread use of ultrasonic contrast agent, a vessel imaging technique and a tumor imaging technique have been developed, and therefore, expectations are rising for improved diagnostic performance.
The bloodstream around a tumor or inside the tumor indicates not only whether any lesion exists, but also properties of its tissue, and provides significant information for a differential diagnosis. Previously, CTA (CT-Angiography) which uses X-rays has been the mainstream of the method for acquiring an image of vessels. In the CTA, an iodine contrast agent is intravenously injected, and a plurality of X-ray images are reconstructed, each of the images being acquired during a passage of the contrast agent through blood vessels, thereby allowing a vascular structure to be three-dimensionally visualized. On the other hand, the CTA sometimes places a load on a patient, due to exposure to X-rays and administration of the contrast agent.
Compared with this, ultrasonic diagnostic imaging is not attended with invasion such as exposure to X-rays when imaging is performed. In addition, since micro bubbles, a few micrometers in diameter, are used as the contrast agent, the contrast agent itself does not possess any toxicity. Such micro bubbles are discharged from the body as time advances, by a natural metabolic function in the body. Therefore, the ultrasonic imaging diagnosis has a characteristic that a burden placed on the patient is small. The micro bubbles (ultrasonic contrast agent) issues strong nonlinear signals, in sympathetic vibration with a few MHz ultrasonic waves used in a medical field. Therefore, it is possible to detect these nonlinear signals specifically to create a picture, thereby visualizing a microscopic vascular structure as a high-contrast image.
The ultrasonic contrast agent is roughly classified into a high sound pressure type and a low sound pressure type, according to a difference in behavior against ultrasonic irradiation. As for the high sound pressure type agent, air bubbles are collapsed by applying pressure under ultrasonic irradiation of high sound pressure (mechanical index: MI from 1.0 to 1.9), and nonlinear signals generated at this time produce an image. Since the contrast agent (micro bubbles) disappears every irradiation, it is necessary to change an imaging plane appropriately in order to observe a contrast image of an identical region. On the other hand, as for the low sound pressure type (MI from 0.1 to 0.9), an image is produced by nonlinear signals which are obtained by establishing resonance under the ultrasonic irradiation, without collapsing the air bubbles by applying pressure. Therefore, a persistent contrast imaging effect is produced, thereby allowing continuous observation of the identical region. In addition, a part of the contrast agent (micro bubbles) is subjected to Kupffer cell phagocyte function, the cells existing in the sinusoid connecting the artery and the portal vein, with the central vein. Therefore, if the ultrasonic irradiation is applied in the state where the liver tissue is filled with the contrast agent, a region where the Kupffer cells are normally functioning is enhanced at high intensity, and it is possible to identify an area of lesion, such as tumor, according to a defect of intensity. Furthermore, an intensity level or duration of contrast enhancement serves as an index for evaluating the function of the Kupffer cells. Therefore, such information items are also considered to be effective for functional diagnosis of the liver.
A contrast image using ultrasonic waves as described above is effective not only for observing the microscopic vascular structure, but also for determining a function of tissue, and it is getting to be used widely, with a focus on an abdominal region.
There is further an advantage that the low sound pressure type contrast agent as described above issues nonlinear signals according to resonance caused by ultrasonic irradiation, and maintains a contrast effect, therefore allowing observation of the contrast enhancement progress in an identical imaging plane.
A process of the contrast enhancement using the ultrasonic contrast agent is different by the types of tissue. For example, in the case of a normal liver, each vessel of artery, portal vein, and vein is enhanced in a different time phase, according to a difference of its neural path, and thereafter, the enhancement of the tissue is performed. However, if a tumor exists, the process of enhancement is different depending on the degree of vascular proliferation or activation. Therefore, by observing dynamics of the enhancement in detail, it is possible to know an aspect of the tumor. Such difference in the enhancement process depending on the tissue indicates a bloodstream variation, i.e., blood flow dynamics, such as a flow passage, a flow amount, and a flow rate of the bloodstream originating from a heart.
Time-Intensity Curve (TIC) serves as a significant index in quantifying a difference in the process of enhancement. This is obtained by plotting intensity change per time unit along with an inflow of the contrast agent. For example, in the case of a liver tumor, it is important to know in discrimination, which is the starting point to form a tumor vessel, from the artery or from the portal vein. However, since the artery runs in parallel with the portal vein, once the enhancement begins in the portal vein, it is difficult to discriminate the portal vein from the artery, by viewing a just single contrast image. However, by comparing the TICs of the respective vessels, a difference in the process of enhancement can be evaluated, and it is possible to objectively determine which vessel is the starting point of the tumor vessel.
The Patent Document 1 describes a technique which calculates an index value such as a mean intensity value, from the TICs measured by a wave transmission sequence where high and low sound pressures are combined, and displays an image which is color-coded according to the calculated value. Ultrasonic irradiation under high sound pressure against the tissue filled with a contrast agent triggers a start of the TIC measurement, and subsequently, the measurement is conducted as to a process of enhancement of the contrast agent with which the imaging plane is reperfused, by the irradiation under low sound pressure.